Variable inductor



June 1970 s. L. DAWSON ETAL 3,518,595

VARIABLE INDUGTOR Filed Oct. 21, 1968 SAM/1054. LEE D4 4306 NoQ/vm/vB02254 RELKMEE INVENTORS p mdazll, L ,Zuz

71 oQ Q United States Patent 3,518,595 VARIABLE INDUCTOR Samuel LeeDawson and Norman Darrel Felkner, Los Angeles, Calif., assignors to WyleLaboratories, El Segundo, Calif., a corporation of California Filed Oct.21, 1968, Ser. No. 768,985 Int. Cl. H01f 21/06 US. Cl. 336-434 6 ClaimsABSTRACT OF THE DISCLOSURE Variable inductors which have high stabilityand resist influence by external magnetic fields, comprising a toroidalcore with an air gap, and an armature of ferromagnetic material whichcan be moved into the gap to bridge it, or out of the gap. A coil woundabout the toroidal core displays an inductance which depends upon theposition of the armature.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to variable inductors.

Description of the prior art A typical variable inductor employs atubular shell with multiple windings of a conductor thereon. Acylindrical armature of ferromagnetic material is mounted to slide inand out along the axis of the tube, to thereby change the inductancedisplayed by the windings. Such types of variable inductors are readilyinfluenced by changing external magnetic fields, and changes in theposition of iron bodies around them. This is due to the fact that thelines of magnetic flux lie partially outside of the windings and ferritearmature. Changes in the immediate environment which alter thereluctance of the path taken by these lines of magnetic flux, or whichadd or subtract magnetic flux at these outside areas can change theeffective inductance.

In order to reduce uncontrolled variations in inductance, prior artvariable inductors have generally required shielding. The shieldingadded weight and cost and made heat dissipation more difficult. Althoughshielding increases stability and freedom from external fields andmaterials, the cylindrically shaped variable inductors were stillsubstantially sensitive to changes in ambient temperature, due toexpansion and contraction of the core in relation to the winding.

OBJECTS AND SUMMARY OF THE INVENTION One object of the present inventionis to provide a variable inductor of maximum stability.

Another object is to provide a variable inductor whose inductance can bevaried over a wide range.

In accordance with the present invention, a variable inductor isprovided which includes a toroidal core of high permeability material,which has an air gap. The core has windings thereabout which display aninductance dependent upon the reluctance of the air gap. An armaturepositioned at the location of the air gap includes a portion of highpermeability such as a ferrite, and a portion of low permeability suchas air or certain non-ferrous metals such as copper. The armature ismounted for movement toward and away from a position where it bridgesthe air gap in the toroid, to thereby change the inductance displayed bythe windings.

In one embodiment of the invention, the ends of the toroid on eitherside of the air gap are flat. The armature comprises a fiat disc whichis almost as thick as the air gap. The disc has 180 of ferrite materialand 180 of 3,518,595 Patented June 30, 1970 a non-ferromagnetic materialsuch as copper or aluminum which is electrically conductive. The disc isrotatably mounted, so that it can be turned from a position wherein thegap is bridged entirely by ferrite material, entirely by copper, orpartially by each. The non-ferromagnetic but electrically conductivematerial enables large changes in inductance, such as 15%, as comparedwith the change between the inductance level when there is a ferrite inthe gap and when the gap is empty.

The use of a toroidal core with a small gap concentrates the magneticflux to positions near the gap, even when a ferromagnetic armature isnot present in the gap. This limits the effect of external magneticfields or changes in the position of ferromagnetic materials in theenvironment, as compared with previous cylindrical variable inductors.The variable inductors are useful in a wide range of applications, suchas in tuned circuits to vary the tuned frequency.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of avariable inductor constructed in accordance with the invention;

FIG. 2 is an exploded, partially sectional perspective view of avariable inductor constructed in accordance with a second embodiment ofthe invention;

FIG. 3 is a partially sectional perspective View of a variable inductorconstructed in accordance with a third embodiment of the invention;

FIG. 4 is a partially sectional perspective view of a variable indutcorconstructed in accordance with a fourth embodiment of the invention; and

FIG. 5 is a perspective view of a variable inductor constructed inaccordance with a fifth embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a variableinductor comprising a core 10 of generally toroidal shape. An air gap 12is formed in the core, and an armature 18 is disposed partially withinthe gap. Multiple windings of an insulated conductor 14 are disposedabout the core, and the inductance displayed by the windings varies inaccordance with the position of the armature 18.

The ends 15 and 16 of the core on either side of the gap are flat. Thearmature 18 is in the form of a short cylinder or flat disc with athickness approximately equal to that of the air gap 12 of the core. Aportion of the armature is in the gap, but its cylindrical axis isoutside of, or displaced from, the gap. The armature comprises a portion20 of a ferromagnetic material (i.e. a material with a permeability atleast several times that of free space) such as a ferrite, and another180 portion 22 of a non-ferromagnetic material (Le. a material with apermeability approximately equal to that of free space) which is also agood electrical conductor, such as copper. The two portions 20 and 22are sectors of the cylinder, so they extend throughout the cylinderlength.

The armature 18 is fixed to a shaft 24 which is rotatably mounted onbearings 26 and 28. The end 25 of the shaft is enlarged to hold it inplace, and a domed or menisius-shaped spring washer 29 is provided topress against the shaft end 30. The washer 29 provides an appreciablebut limited resistance to turning of the shaft 24, to maintain thearmature in any position to which it is turned. The end 30 of the shafthas a slot 31 for receiving a screwdriver to facilitate turning of theshaft.

The entire apparatus may be potted with resin or the like to provide acomplete package. In the position shown in FIG. 1, the entire gap isbridged by the ferromagnetic portion 20, thereby providing a minimumreluctance across the gap and a maximum inductance for the device. Ahalf rotation of the armature places the non-ferromagnetic materialentirely within the gap 12. This increases the reluctance of the gap,thereby decreasing the inductance to a lower level.

The non-ferromagnetic material of portion 22 has a permeability almostequal to that of free space. Thus, the static reluctance of the pathacross the gap in the core is reduced to that existing when nothing isin the gap. However, the electrical conductance of the material ofportion 22 results in the induction of currents therein when themagnetic flux in the core is changing. These currents induced in thematerial at 22 oppose the change in flux, and result in an effectivedecrease of the reluctance of the air gap to an even smaller level thanexists for free space. This effect is very noticeable when the windings14 carry high frequency currents, and enables a decrease of inductanceto a low level. For example, at a frequency on the order of 1 mHz. thechange in inductance achieved by turning the armature 180 can be on theorder of :15 when the portion 22 is of a good conductor such as copper.If free space is substituted for copper, the change is only about '-10%The toroidal variable inductors are relatively insensitive to externalmagnetic fields or bodies, and have been found to display good stabilityunder changes of temperature. The path of flux is of low reluctance, sothat the device is efiicient and a large inductance is provided in asmall volume. In addition, a high Q is realizedso that a largeinductance is provided with a low resistance of the windings.

Toroidal cores generally must be wound in special machines whichrepeatedly thread a conductor through the hole in the core. However, inmany cases the toroidal cores of the invention can be wound by allowingthe wire to pass through the air gap during winding. This can reduce thecost of constructing the variable inductors and enable more turns to bereceived.

FIG. 2 illustrates a second embodiment of the invention wherein the core40 has concavely rounded depressions 42 and 44 of part-cylindrical shapeon either side of the gap 46 in the core. The depressions face in thesame direction and are aligned with each other, and each depressionfaces substantially perpendicular to the other end of the core. Anarmature 48 of elongated cylindrical shape is disposed in thedepressions to bridge the gap. The armature has a portion 50 offerromagnetic material defining a half cylindrical or 180 portion, and aportion 51 of non-ferromagnetic material comprising the other 180 of thecylinder. Each portion 50 and 51 extends throughout the cylindricallength of the armature, although each comprises only a portion of thearmature cross-section.

The armature 48 is mounted in a pair of bearings 52 and 54 at eitherend, and a slotted boss 56 is fixed to one end of the armature tofacilitate turning. Turning of the armature brings all or part of theferromagnetic portion 50 closely against the surface of the core at thedepression, to cause variations in the inductance of the device. Theembodiment of FIG. 2 is especially resistant to external fields orbodies because the entire ferromagnetic portion 50 is always arelatively small distance from the depressions 42 and 44 in the core.The limited distance assures that nearly all flux lines not passingthrough the non-ferromagnetic portion 51, will pass through theferromagnetic portion 50, rather than through any iron or other bodiesin the environment.

FIG. 3 illustrates a third embodiment of the invention which comprises aferromagnetic core 60 with a gap 62, and annular or rounded depressions64 and 66 on either side of the gap. The depressions 64 and 66 face eachother to closely surround a cylindrical armature 68 on opposite annularsides thereof. The armature has a cylindrically shaped portion 70 offerromagnetic material and another cylindrically shaped portion 72 ofthe same size of non-ferromagnetic material, the two portions disposedend-to-end and in alignment.

The armature 68 is constrained to movement in and out of the gap 62 inthe core by a plastic tube 74 surrounding the armature. A rod 76 joinedto the portion 70 can be pushed or pulled to move the armature parallelto its cylindrical axis. Of course, the inductance is greatest when theferromagnetic portion 70 is fully within the gap, and is least when itis fully withdrawn and the nonferromagnetic portion 72 is therein. Thisembodiment enables the direct conversion of a linear motion to a changein inductance, as is the case in the variable inductors known heretoforewhich comprised a cylindrical winding. However, the embodiment of FIG. 3is more stable, more insensitive to external fields and bodies, and morecompact than the prior art types.

FIG. 4 illustrates a fourth embodiment of the invention which utilizes athreadably mounted armature 80' to vary the gap 82 in a toroidal core84. One end 86 of the core has a rounded depression partially encirclingthe armature, and the other end 88 is fiat. The armature, which isconstructed entirely of ferromagnetic material, is of cylindrical shapewith a flat end 89, to closely engage the ends of the core. The armatureis fastened to a threaded member 90 with a slotted end 92, which isthreadably engaged with a nut-like member 94. Rotation of the threadedmember 90 moves the armature end 89 toward and away from the flat end 88of the core, to-

increase and decrease, respectively, the inductance displayed by thecore winding 96.

The embodiment of FIG. 4 enables changes in inductance to continuethrough several turns of the armature which can facilitate fineadjustments. In addition, the tendency of the core to move the armatureto the position of greatest inductance is resisted by the threadablemounting, even for relatively course threads. The ends 88 and 89 of thecore and armature, respectively, are preferably mated to enable a largearea of contact when moved together, to provide a high maximuminductance. However, instead of flat ends, other shapes such as taperedconfigurations can be employed to vary the manner of change ofinductance.

FIG. 5 illustrates a fifth embodiment of the invention wherein thearmature is of short cylindrical shape with a band-like portion 102 offerromagnetic material and two portions 104 and 106 of non-ferromagneticmaterial on either side. The armature is rotatably mounted along itscylindrical axis 108, by a rod and bearings (not shown). This embodimentof the invention enables a variation from maximum to minimum inductancewith only a quarter-turn of the armature.

While each of the foregoing embodiments of the invention have peculiaradvantages, all of them have the advantages resulting from the toroidalshape with an armature which can substantially completely close thelimited gap in the toroid. These include the reduced sensitivity toexternal fields and bodies, and increase stability, compactness, andease of winding. While toroids of substantially constant rectangularcross-section are showing a variety of toroidal-like shapes can beutilized, so long as the core defines a substantially closed path forthe flux except for a gap which can be closed by the movable armature.

Variable inductors have been constructed in accordance with the above bywinding a conductor around a core and molding or potting them in anepoxy. Many variable inductors of the types shown in FIGS. 1 and 2 havebeen constructed utilizing a ferrite for the ferromagnetic portions ofthe core, and copper or aluminum for the non-ferromagnetic portions.Small variable inductors with an outside diameter of 4 inch, and airgaps approximately inch long were constructed with inductances ofmicrohenries to 1 millihenry. The inductors were utilized at frequenciesof 100 kHz. to 25 mHz., and displayed changes in inductance on the orderof +-IS%. The inductors operated stably without the necessity formagnetic shielding or close control of the environment.

Although particular embodiments of the invention have been described andillustrated herein, it is recog nized that modifications and variationsmay readily occur to those skilled in the art, and consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is:

1. A variable inductor comprising:

a toroidal-like core of ferromagnetic material, said core having endsforming a gap in said core;

a winding disposed about said core to generate magnetic flux therein;

an armature comprising a first portion of ferromagnetic material, and asecond portion of nonferromagnetic material which is of high electricalconductivity; and

means for positioning said armature in a plurality of positions,including a first position wherein said ferromagnetic portionsubstantially completely bridges said gap while substantially none ofsaid material of high electrical conductivity is in said gap, and asecond position wherein said material of high electrical conductivitysubstantially completely bridges said gap while substantially none ofsaid ferromagnetic material is in said gap.

2. The variable inductor described in claim 1 wherein:

said ends of said core at said gap are substantially flat and parallelto each other;

said ferromagnetic portion of said armature comprises a portion of adisc which has a thickness approximately equal to the separation of saidends of said core and an axis displaced from said gap; and

said means for positioning said armature comprises means for rotatingsaid ferromagnetic portion about said disc axis and maintaining it inany of a plurality of rotational positions thereabout.

3. A variable inductor comprising:

a toroidal-like core of ferromagnetic material, said core having endsforming a gap in said core, each of said ends having trough-like concavedepressions which face in the same direction;

a winding disposed about said core to generate magnetic flux therein;

an elongated cylindrical armature with its rounded perimeter mated toand disposed within said concave depressions, said armature including aferromagnetic portion thereof extending throughout the length of saidcylinder along only a portion of the cross-section thereof; and I meansfor rotating said armature about its cylindrical axis.

4. A variable inductor comprising:

a toroidal-like core of ferromagnetic material, said core having endsforming a gap in said core, said ends including a first end having atrough-like depression and a second end in line with an imaginaryextension of said trough-like depression;

a winding disposed about said core to generate magnetic flux therein;

an armature comprising a substantially cylindrically shaped portion offerromagnetic material for sub stantially bridging said gap, to providea substantially closed magnetic path through said core; and

means for threadably mounting said armature for rotation about the axisof its cylindrically shaped portion to advance it toward and away fromsaid second of said end of said core while maintaining it in constantengagement with said first end of said core.

5. A variable inductor comprising:

a toroidal-like core of ferromagnetic material, said core having endsforming a gap in said core, each of said ends having a trough-likedepression;

a winding disposed about said core to generate magnetic flux therein;

a substantially cylindrical armature having a substantially strip-likeportion of ferromagnetic material extending substantially diametricallythrough said armature and along its length to substantially bridge saidgap in said core, said armature also having portions on diametricallyopposite sides of said cylinder which are free of ferromagneticmaterial; and

means for rotating said armature about the axis of said cylinder, saidstrip-like portion being thin enough so that none of it is directlyopposite either of said core ends of at least one position of saidarmature, whereby the inductance changes between a maximum and a minimumwith a substantially quarter-rotation of said armature.

6. The variable inductor described in claim 5 wherein:

said portions of said armature which are free of ferromagnetic materialare constructed of nonferromagnetic material of high conductivity.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 3 1939' Austria.11/ 1956 Germany.

1948 Great Britain. 875,468 8/ 1961 Great Britain. 239,091 12/ 1945Switzerland.

THOMAS J. KOZMA, Primary Examiner U.S. Cl. X.R.

